1. Field of the Invention
The present invention relates to a method for controlling the temperature of a heated roll in a calender, the calender comprising at least one heated roll or thermoroll and at least one backing roll, between which is formed a nip, through which the material web is conveyed, and to which heated roll or thermoroll is supplied internal thermal power by internal heating means of the roll or thermoroll, and external thermal power by external heating means of the roll or thermoroll.
2. Description of Related Art
Previously are known thermorolls or heated rolls which are used in the finishing of, for example, paper or other similar material, especially for improving/altering the quality of a material web formed of paper. In basic operation, heat or thermal power is supplied to the thermorolls continuously to heat the mantle of the thermoroll and maintain it at a constant temperature. Through the outer shell of the roll, that is, the mantle, the thermal power corresponding to the desired grade of the web material is transferred to the paper web. The thermal power correlates with the temperature of the roll mantle surface.
Also previously are known two methods for heating the roll, which differ in basic principle. The first uses internal heating, where heated heating medium such as water, steam or oil is fed inside the roll. The second method uses external heating, applying external heating means. Such heating means can be, for example, induction heating elements arranged axially in the vicinity of the roll mantle, blasting elements or elements intended for infrared heating.
One structure of an induction heating element is described in the U.S. Pat. No. 5,895,598, which concerns placing inductive elements also inside the roll to effect more even distribution of heat inside the roll.
The roll may also be heated with external and internal heating means simultaneously. Finnish patent application 882865 discloses an arrangement of this type, the purpose of which is to maintain, by means of additional external heating means, as accurately as possible a temperature balance in the radial direction of the roll. The balance is susceptible to change, especially at a moment when the transfer of the thermal power supplied from the roll mantle to the web material changes. Such moments include start-up, stopping, and change of grade of the web material.
The problem with such rolls concerns long heating and cooling times due to the large mass of the roll. When a thermoroll is heated with oil, the roll may be heated or cooled preferably by about 1 to 3 degrees centigrade per minute. When the grade of the material web is changed during running, the production of the web grade must be discontinued at the moment when the roll temperature dependent on the thermal power transferred from the roll to the web material changes. A new grade cannot be run until the temperature of the roll surface (mantle), in practice, the temperature of the entire roll, has changed to correspond to the new grade. The same problem occurs when the heated roll is cooled before changing.